[Solved] JavaScript Error: Unable to get property 0 of undefined or null reference

Please help I wanted to use this code to validate marks enter and also, to tick the value enter using javascript but I have successfully not been pleased help.

I wanted upon entering a score the checkbox is ticked using this very code.

function GetSelectedRow(UserLink) {
    var row = UserLink.parentNode.parentNode;
    var rowIndex = row.rowIndex - 1;
    var userId = row.cells[0].innerHTML;
    var lable = row.getElementsByTagName("input")[2].value;
    if (lable == null || lable == "") {
        row.getElementsByTagName("input")[8].value = "d";
        return true;
    }
    else
        row.getElementsByTagName("input")[8].value = "";

    if (lable >= 0 && lable <= 100) {
        return true;
    }
    else {
        alert("Must be between 1 and 100");
        return false;
    }
}

I am getting error at line 4

var userId = row.cells[0].innerHTML;

It flags an error

Unable to get property '0' of undefined or a null reference

<asp:GridView ID="GvStudentMarks" runat="server" AutoGenerateColumns="False" OnRowDataBound="gvStudentMarks_RowDataBound">
    <columns>
        <asp:TemplateField>
            <HeaderTemplate>
                <asp:CheckBox ID="chkAll" runat="server" AutoPostBack="true" />
            </HeaderTemplate>
            <ItemTemplate>
                <asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="true" />
            </ItemTemplate>
        </asp:TemplateField>       
        <asp:TemplateField HeaderText="Eng">
            <ItemTemplate>
                <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                    <ContentTemplate>
                        <asp:Label ID="lblEng" runat="server" Text='<%# Eval("Eng") %>' Visible="False"></asp:Label>
                        <asp:TextBox ID="TxtEng" runat="server" OnTextChanged="txtEng_TextChanged" Text='<%# Eval("Eng") %>' onkeyup="enter(this)" AutoPostBack="True" Height="16px" Width="58px" onchange="return GetSelectedRow(this)"></asp:TextBox>
                        <asp:RangeValidator ID="RangeValidator1" runat="server" BackColor="#FF99FF" ControlToValidate="TxtEng" Display="Dynamic" ErrorMessage="Invalid" MaximumValue="100" MinimumValue="0" SetFocusOnError="True" Type="Double"></asp:RangeValidator>
                    </ContentTemplate>
                </asp:UpdatePanel>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Maths">
            <ItemTemplate>
                <asp:UpdatePanel ID="UpdatePanel2" runat="server">
                    <ContentTemplate>
                        <asp:Label ID="lblMaths" runat="server" Text='<%# Eval("Maths") %>' Visible="False"></asp:Label>
                        <asp:TextBox ID="TxtMaths" runat="server" OnTextChanged="txtMaths_TextChanged" Text='<%# Eval("Maths") %>' onkeyup="enter(this)" AutoPostBack="True" Width="48px" onchange="return GetSelectedRow(this)"></asp:TextBox>
                        <asp:RangeValidator ID="RangeValidator2" runat="server" BackColor="#FF99FF" ControlToValidate="TxtMaths" Display="Dynamic" ErrorMessage="Invalid" MaximumValue="100" MinimumValue="0" SetFocusOnError="True" Type="Double"></asp:RangeValidator>
                    </ContentTemplate>
                </asp:UpdatePanel>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="TotalMarks">
            <ItemTemplate>
                <asp:Label ID="lblTotalMarks" runat="server" Text='<%# Eval("avgs") %>' Visible="False"></asp:Label>
                <asp:TextBox ID="txtTotalmarks" runat="server" Text='<%# Eval("Avgs") %>'></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Average">
            <ItemTemplate>
                <asp:TextBox ID="txtAverage" runat="server" Text='<%# Eval("Avgs2") %>'></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
    </columns>
</asp:GridView>